EFFECTS MOISTURE REGIME, AMOUNT APPLIED AND VARl ETY THE RIPENING NITROGEN RESPONSE OF SUGARCANE TO GLYPHOSATES

Results from 30 trials were used to test the effect of moisture regime (rainfall and irrigation), level of nitrogen fertilizer or variety on the ripening response to glyphosate. The results showed that adequate moisture was required at the time of spraying and between spraying and harvesting to produce good ripening responses. At least five hours should elapse between spraying and heavy rainfall or irrigation, however, varieties appear to react differently. The level of nitrogen applied did not affect the response to the ripener but did affect both cane yield and quality. The current recommendations on timing and level of fertilizer application appear to be adequate to ensure that good ripening responses are obtained with glyphosate. Varieties N52/219 and N8 appear to be more susceptible than other varieties to early chlorosis and stunting in the following ratoon crop. The full effects on ratoon regrowth will only be known once the subsequent ratoon crop has been harvested. Practical implications of the results are discussed. Introduction Results from recent trials in South Africa have shown (Clowes4~ that glyphosates were effective in improving cane quality during the growing period in each of the three main ecological regions, while during the cooler ripening period responses were only obtained in the northern semi-arid region where cane is irrigated. Good ripening responses from Polaris, a chemical ripener related to glyphosate, were obtained on clay but not on sandy soils in Hawaii (Mason9). It was suggested that the poor ripening response on sandy soils was the result of the poor soil water holding capacity, which favoured natural ripening. The nitrogen content of leaves has also been shown to be associated with ripening responses. In addition, all varieties treated with glyphosate have responded over a wide range of climatic conditions experienced in Hawaii (An0n.l). Glyphosates sometimes increase tillering of the following ratoon crop, ( A n ~ n . ~ ) probably as a result of inhibition of of indole acetic acid (IAA) transport (Baur3), and may even cause chlorosis and early stunting (Clowes4), particularly if higher rates are used. The effects on regrowth are complex and not fully understood. However, Greshoff6 found that inhibition of crop regrowth by glyphosates was transitory, except in plant roots, and that phenylalanine and tyrosine acted synergistically to reverse this growth inhibition. The inhibition of growth probably results from the interference of glyphosate with aromatic amino acid metabolism (Jaworski8; Haderlie, Widholm and Slife7). The effects of different management practices on the response to growth regulators needs to be ascertained under local conditions because these effects are likely to depend on complex genotype: environment interactions. This paper is a report of the results from 30 trials designed to provide information on the effects of soil moisture regime, level of applied fertilizer and sugarcane variety on the ripening response to glyphosate. The glyphosate salts used were either Roundup at a rate of 1 litre per hectare, or Mon 8000 applied at a rate of 0,67 kg per hectare. These treatments both represent an application of approximately 0,4 kg active ingredient per hectare. Materials and Methods Details describing each trial, excluding the 20 trials at Tongaat, are shown in Table 1 and more detailed information on each trial is presented below : Trilal I at Pongola: was designed to determine the effects of glyphosate on cane which was either irrigated or not-irrigated after the application of the ripener. Two 3,2 hectare blocks received different irrigation treatments while eight 0,4 hectare sub-plots within each block were divided into four replicates, each one comprising a treated and an untreated sub-plot. Figure 1 shows the design, and the dates of irrigation, ripener application, sampling and harvesting, and the daily rainfall. TABLE 1 Description of trials (excluding the series of 20 trials a t Tongaat) * For variety NCo 376 t NI = Northern irrigated; C = Coastal; M = Midlands $ P = Plant crop; R = Ratoon crop Trial